Hayden Rapid-Cool transmission cooler review

The Rapid-Cool 678 is a tube-and-fin cooler measuring 3/4 by 11 by 9-1/2 inches, built with patented internal turbulation that Hayden states increases heat transfer with minimal pressure drop. Hayden sizes it for pickup trucks, vans and Class C motorhomes and travel trailers up to 24 feet, rated to a GVWR up to 24,000 lb and towing up to 5,000 lb. Hayden Automotive has built coolers in this category for decades, which is part of why this specific model shows up so often as a first cooler.

All four sit in the transmission cooling line outside the radiator. What actually separates the Rapid-Cool from the rest of this table is price and core design, not whether it does the same fundamental job.

Hayden's own listing states the Rapid-Cool 678 is an add-on cooler rather than a direct replacement for the factory unit, so it installs in series with what is already there instead of taking its place. That is a smaller, simpler job than replumbing a vehicle's only cooler, but it also means the factory cooler's own limitations, whatever they are, are still part of the system.

The core design is the other tradeoff. Tube-and-fin construction moves less heat for the same frontal area than the stacked plate design used in the Mishimoto 13-row, which is the reason the Rapid-Cool costs a fraction of that cooler's price. For a half-ton truck making occasional short tows, that difference rarely matters. For a truck climbing long grades at or near its tow rating repeatedly, it can.

A reader towing heavier or climbing longer grades more often should step up to the Mishimoto 13-row stacked plate cooler, rated to 40,000 lb GVW with substantially more core volume, though as a universal kit rather than an add-on sized to a stated towing figure. A reader whose factory cooler on a Ford has already failed should compare the Dorman replacement cooler instead, built to match the original part's function rather than add capacity. A Ram Cummins owner doing heavy towing should look at the Mishimoto direct-fit Ram Cummins cooler, which replaces the stock cooler outright rather than adding to it.

Confirm the towing and GVWR figures Hayden publishes actually cover the vehicle and trailer combination before assuming any add-on cooler is enough, using the long grade towing guide to understand when transmission heat actually peaks. Check the grade and descent gear chart for the gear selection that keeps heat down on the climb, and compare this add-on against a higher-capacity universal kit in the Mishimoto transmission cooler review before deciding which fits the actual towing done.